While I am not fully convinced that Ruby is the best language ever and that Rails rules, I think RoR (Ruby on Rails) is great for prototyping and maturing webapps.
Setting up Ruby and Rails for development on a Mac is not as easy as with Windows but it's mostly just a sequence of the usual *nix get-expand-configure-make-install dance step. My recommendation is to avoid the easy alternatives like Fink, DarwinPorts, and Locomotive. I've heard great things about the first two but I think they have since fallen behind far enough to be inappropriate for leading edge development works. Locomotive is better and flexibility offered by bundling is great but I found their setup awkward and, well, I have little need for bundle silos.
To setup my MBP (running OS X version 10.4.8) for RoR development environment, I followed the instructions in Building Ruby, Rails, LightTPD, and MySQL by Dan Benjamin but using the latest versions of packages and also installing rake and mongrel gems. Afterward, I upgraded Rails to 1.2 RC1 to give its REST support a spin.
Re IDE, both TextMate (~$50) and RadRails (free) are great for RoR development on OS X.